130 



BOTANY 



these are several rows of rather loosely placed cells, called collectively the 

 spongy parenchyma. These cells do not contain so much leaf green as 



those of the palisade layer. Notice the 

 spaces between these cells. In some cases 

 they can be seen to communicate with the 

 openings of the stomata in the lower epi- 

 dermis. Look for the cut ends of one or 

 more of the fibro vascular bundles or veins. 

 The cells of which they are composed are 

 seen to be greatly thickened.^ 



Cross Section of a Green Leaf. — 

 The leaf in cross section shows that the 

 whole blade is a series of tissues, the epi- 

 dermis being somewhat thicker walled for 

 protection, the under surface pierced by 

 numerous pores. Each of these pores con- 

 nects with an air space which penetrates 

 more or less the whole inside of the leaf, 

 but especially the layer just outside the 

 lower epidermis. The cells of the palisade 

 layer we shall now consider more in detail. 



Section of a leaf; e, epidermis; 

 c, cells containing chlorophyll 

 granules; p, intercellular pas- 

 sages; g, g, guard cells of stoma. 



Chloroplasts. — If we examine some 

 of the plant cells forming part of the 

 blade of the leaf, we find cells which are almost cylindrical in 

 form. In the protoplasm of such cells are found a number of 

 little bodies colored green, which are known as chloroplasts or 

 chlorophyll bodies. If we place the leaf in wood alcohol, we 

 find that the bodies still remain, but that the color is extracted, 

 going into the alcohol and giving to it a beautiful green color. 

 The chloroplasts are, indeed, simply part of the protoplasm of 

 the cell stained green. If the plant is kept in the sun, the chloro- 

 plasts keep their green color, but in the dark this color is gradually 

 lost. These bodies are of the greatest importance directly to 

 plants and indirectly to animals. The chloroplasts, by means of 

 the energy received from the sun, manufacture starch out of certain 

 materials. These materials are soil water, which is passed up 

 through the fibrovascular bundles into the veins of the leaf from 

 the roots, and carbon dioxide, which is taken in through the sto- 

 mata or pores, with which the under surface of the leaf is covered. 



* For detailed laboratory exercises, see Hunter and Valentine, Maniml, pages 

 47-49. 



